This invention relates to an improved process for preparing anhydride polymers, notably poly(maleic anhydride). In particular, this invention relates to the use of amines during polymerization that allow more efficient use of freeradical initiators in achieving high monomer conversion of maleic anhydride.
Low molecular weight hydrolyzed poly(maleic anhydride) polymers and its salts are well-known as dispersants, scale inhibitors, detergent additives and sequestrants; generally a molecular weight below about 2,000 is typical for these applications. Previous processes for preparing poly(maleic anhydride) using aromatic hydrocarbon solvents, such as toluene and xylene, used large quantities of free-radical initiators but the final polymers contained substantial amounts of unpolymerized maleic anhydride. Attempts to improve the efficiency of polymerization include the use of di-tert-butyl peroxide initiator in xylene and temperature dependent isolation of poly(maleic anhydride) as an immiscible phase (U.S. Pat. No. 3,919,258), use of peroxyester initiators (U.S. Pat. No. 4,818,795) and dilute solution polymerization in xylene with limited initiator concentrations (U.S. Pat. No. 5,077,364). However these approaches still do not combine efficient use of readily available initiators with high monomer conversion.
The present invention seeks to overcome the problems associated with prior art processes used to prepare poly(maleic anhydride) by providing an efficient polymerization process using low levels of initiator while providing high monomer conversion.